what a Captcha routine does is adds an image to the form that the submitter must hand type in for the form to submit.
if the submitter doesn't get it correct the form will not submit.

This will help eliminate the ever growing use of other server based form submissions that spammers are using.
it should also cut down on the frequent spammer that just goes around and submits forms one after another, i say should because there will still be idiot spammers that will hand type in the proper code to submit the form.

a Captcha example is would look like this:

The Captcha portion is this: